The Fibre Channel (FC) standards are an American National Standards Institute (ANSI) standards set that defines a common transport system for use with different protocols or applications. The initial core FC standard is identified as X.3230-1994-Fibre Channel Physical and Signaling Standard (FC-PH), the disclosure of which is incorporated by reference herein.
Until recently, FC optical transceivers were specified with an acceptable range of optical receiver sensitivity and transmitter output measured in terms of average optical power at a minimum extinction ratio level. When an optical link failure occurred, fault determination procedures typically called for measurement of the average optical power at both transmit and receive ends of the link. If the transceiver power levels were within specified limits, but the link continued to fail, then the problem was attributed to the optical cable plant. It was possible to measure segments of the link using an average optical power meter until the fault was localized and corrected. On the other hand, a defective transceiver could be identified by measuring its optical output and input power levels. If the link was delivering acceptable power levels and errors continued to occur, the failure was assumed to reside in the optical receiver.
Recently, however, the FC standard changed its specifications. The change is found in FC-PI-2, Revision 4, Appendix A.5, the disclosure of which is incorporated by reference herein. Rather than including receiver sensitivity at a given extinction ratio, the Standard now defines a new parameter, Optical Modulation Amplitude (OMA), which refers to the optical amplitude of the signal, i.e., the difference in amplitude between a logic 1 and a logic 0. The OMA is a function of both received average optical power, PAVE, and extinction ratio, E:
                    ⁢                  OMA        Linear            =              2        *                  P          AVE                *                  (                                    E              -              1                                      E              +              1                                )                ⁢        in        ⁢                                  ⁢        linear        ⁢                                  ⁢        units              ⁢                  or            OMA      log        =          10      *      log      ⁢                          ⁢      10      ⁢              (                              2            *                          P              AVE                        *                          (                                                E                  -                  1                                                  E                  +                  1                                            )                                0.001                )            ⁢                          ⁢      in      ⁢                          ⁢      units      ⁢                          ⁢      of      ⁢                          ⁢      dBm      ⁢                          ⁢      with      ⁢                          ⁢              P        AVE            ⁢                          ⁢      in      ⁢                          ⁢              Watts        .            
The extinction ratio is the linear ratio of optical power between a logic level 1 and 0 measured under fully modulated conditions. This Standards change means that it is no longer possible to determine whether an optical transceiver is within specification simply by measuring the average optical power input to the receiver. The OMA, as specified in the Standard, requires measurements of an extinction ratio that, in accordance with existing measurement approaches, can only be made accurately in a lab or manufacturing environment with an expensive digital oscilloscope or similar equipment.
This situation currently affects all FC components, as well as IBM Corporation's zSeries™ Fibre Connection (FICON) links (which use the FC physical layer). Also, the practice of using OMA-based specifications already exists in the 10 Gigabit/second Ethernet (10G Ethernet) Standard, the disclosure of which is incorporated by reference herein, as will be explained below. The OMA-based specifications are likely to extend into higher data rate standards or higher fiber count links in the future, as well as other protocols besides FC and 10G Ethernet.
Since the two standards specify an OMA measurement technique that is quite difficult and expensive to perform in the field by installation/repair personnel, it would be desirable to have a low cost portable OMA measurement tool which can be correlated with the various standards.
The 10G Ethernet Standard specifies a relative intensity noise (RIN) OMA measurement using a photodetector and a power meter. Such an approach is a ratio measurement of noise power to signal power and thus does not calibrate the photodetector to obtain a true OMA reading.
Hewlett Packard (Palo Alto, Calif.) produced the 8151A Optical Pulse Power Meter which was capable of measuring OMA on square wave signals up to 250 MegaHertz (MHz). This unit used separate high and low peak detection circuits with variable ramps to determine the high and low optical levels.
U.S. Pat. No. 5,850,409 and U.S. Patent Application No. 2003/0090289A1 both describe a method and circuit for “measuring OMA.” But, in actuality, these methods and circuits are configured for maintaining the internal OMA of a transmitter over temperature after the transmitter has been appropriately set up by a user. These methods and circuits do not and can not report a calibrated OMA.